EVOH resins are thermoplastic resins prepared by saponifying an ethylene-vinyl ester copolymer, which is a copolymer of ethylene and a vinyl ester monomer, and are excellent in transparency, gas barrier property (e.g., oxygen barrier property), fragrance retaining property, solvent resistance, oil resistance, mechanical strength and other properties. Therefore, the EVOH resins are generally formed into films, sheets, bottles and the like, which are widely used as packaging materials such as food packaging materials, pharmaceutical product packaging materials, industrial chemical packaging materials and agricultural chemical packaging materials.
In these applications, contamination with colored foreign matter, occurrence of fisheyes and the like are considered problematic. The EVOH resins are susceptible to thermal degradation. When such an EVOH resin stagnates in a forming apparatus in a heat melt forming process, for example, the EVOH resin is liable to be thermally degraded. Thermally degraded pieces of the EVOH resin cause the colored foreign matter and the fisheyes in a formed product when entering the formed product.
Various studies have been conducted to solve this problem. One proposed solution is to use a radical initiator having a half-life of not longer than 2 hours at 60° C. as a polymerization initiator for preparation of an ethylene-vinyl ester copolymer that is to be saponified for production of the EVOH resin (see, for example, PTL 1).